Beating the sterile brome menace

Many tillage farmers across the country have been exposed to the threat of sterile brome in recent years. The grass weed normally starts in the hedges and works its way gradually outwards into fields. It is a real threat in non-inversion tillage systems and is an increasingly visible problem in ploughed fields.

There are three herbicides with specific activity for use in wheat crops – Pacifica, Broadway Star and Monitor. All have potentially good activity but none guarantee perfection. The former and latter certainly benefit from warm growing conditions around application timing – a characteristic that makes timely control more challenging.

There are no control or containment options for use in winter barley or oats, so these crops present an added challenge where a rotation is practised. Generally, this weed is not a problem in well managed spring crops and it can be controlled in broad-leaved crops using graminicides.

But why are we depending on chemicals to control this weed problem? Is sterile brome a badge of bad farming? This could be said, but it is also a problem that is not given enough thought.

There is a clue in its name – ‘‘sterile’’ brome. Its seeds are certainly not sterile but, in the past, it was thought to be sterile because seeds that were shed in one crop seldom reappeared as plants in the following crop post ploughing.

There are a number of different brome grasses in tillage fields around the country and some are even present in pastures. Sterile brome is the most dominant, but soft brome and meadow brome are also present in fields. These latter types differ in appearance but all have similar biological characteristics that can be used against them to help provide cultural control.

Sterile brome flowers between May and July and seeds begin to shed shortly after, until after harvest. Germination then takes place between July and November and this is why it is so seldom a problem in spring crops. Meadow and soft brome have a similar emergence pattern but soft brome can also germinate in March and April.

In general, dormancy levels in seeds are very low, so the majority of seeds germinate at the first opportunity. But a small proportion of seeds may carry dormancy and this governs the cultural practices used to help control the weeds. Plants are described as biennial because they seed in the year after establishment.

Taking seeds from light into darkness by covering with chopped straw or through shallow cultivation will help them to germinate. And where a germinated seed is killed by glyphosate or cultivation, it is removed from the weed bank. Dry conditions in the autumn may delay or prevent autumn germination but, in this climate, there will generally be a strike before planting time.

For all of these reasons, problem fields tend to be associated with non-inversion tillage, poor ploughing and early drilling. Good ploughing is still an effective cultural control and later planting helps to significantly reduce numbers. Shallow autumn cultivation also helps to get shed seeds growing early and these can subsequently be killed by either ploughing, good cultivation or spraying with glyphosate.

Spring cropping is a good way to reduce or eliminate infestation, as is the use of broadleaved winter crops sprayed with graminicides. A non-cereal winter break crop provides a total herbicide option using alternative chemistry, while a spring crop allows for an autumn fallow and stale seedbed to grow out the previously shed seeds.

Bury seeds

The ‘‘sterile’’ in its name is not a result of sterility in the weed itself. It is much more a consequence of normally low dormancy levels, combined with the fact that it cannot emerge from depth. So, good ploughing acts as a control process and most of the problems associated with plough-based systems relate to areas where you get incomplete burial or just bad ploughing.

Good ploughing generally controls the problem as seeds do not like being buried, even for one year. The major problem spots tend to be areas where plough performance is poor.

Ploughing implies complete burial of thrash but this is not possible on every square metre.

The main problem areas are:

For these reasons, sterile brome also appears to be a growing problem under the plough.

But what can you do about it? The plough provides the potential for control but only if thrash burial is good. The combination of inversion and non-inversion can be synergistic. In problem fields, leave well-ploughed land unploughed for a year or two to help deplete soil seed reserves – leave the problem buried.

The different problems relating to ploughing necessitate different solutions. Leave areas of loose or stony soil unploughed for a year to help them turn better when ploughed for improved burial after a year. Success is based on organising the field to secure good thrash burial in all affected areas.

By the hedge

The main reason for bad brome beside hedges is because it is nearly impossible to get good burial of the last run of the plough by the hedge when the headland is turned into the field.

The problem in most fields started in the hedges when all other vegetation was burned off to help prevent different problems at that time. And this weed is an opportunist.

To get good control of this weed by the hedge requires one or two actions. Either you turn the headland into the hedge for two consecutive years to help get good burial or you plant a strip of grass and keep it topped to prevent seed return in this area for the same period.

Good burial with the plough is key to successful long-term control.

Where good skimming is practised, seeds will fail to establish in that year or in the following year. But this cannot work if you do not have good skimming.

An alternative solution is to plant a 2m grass buffer strip by the infested hedge. This area will have to be topped to prevent seed return for two to three seasons. You cannot win the battle against brome as long as seed return continues.

After two to three years, you should plough cleanly to bury any seeds still on the surface but do not plough in the following season to minimise the risk of turning any dormant seeds back up again.

Ins and outs

The ins and outs are a major problem area for brome for the same reason – inadequate burial.

If you move your plough headlands out 3m or 4m for two or three consecutive years, you should be able to plough down your problem.

Two to three years of good burial can banish a problem but you must achieve good burial, plus a small requirement for rogueing or limited chemical control.

So far, it seems that brome is not a strongly persistent weed, so if you can grow out a seed bank, the problem will go away. Good burial of the ins and outs for a few years should get rid of the problem.

Out in the fields

In most fields, sterile brome started as a single plant. If that had been pulled or destroyed, a problem may not have developed. With so much of this grass around now, combines tend to be a major method of spread, both within and between fields.

The awned seeds get caught in the various crevices in the combine and only work their way free in time, so they are unpredictable in terms of where they are deposited.

Most sterile brome problems start in the hedge and then gradually migrate into the field.

The majority of seeds continue to be destroyed in a plough-based system but patches occur where the sods will not turn properly or it can quickly show increased numbers in min-till systems.

Where this weed is spread throughout a field, the control options include the use of herbicides in wheat, the use of graminicides in broadleaved crops or a reversion to spring cereals.

Chemical control is both expensive and unpredictable. Cultural control can be good when you use it properly to achieve good burial.

But you need to be able to plough and not just blacken the soil. For many, the art of good ploughing is long lost, so such cultural systems will have limited potential.

For the plough to work, it must achieve total burial. This applies whether by the hedge or out in the field. Good burial is more difficult to achieve where soil is loose or stony and these are the areas where this weed appears in ploughed fields.

Even good ploughing cannot provide good burial in these areas because the soil will not turn cleanly. In these situations, you need to leave the land unploughed for a year to enable it settle tighter, so that it will turn properly when ploughed to provide good burial.

Prevent seed return

The key to successful brome control is to prevent seed return to the soil.

This is easy if the plants do not grow initially but it is not uncommon that a few might survive even where herbicides were used.

To get effective control of a bad problem in a concentrated patch, you must be prepared to either rogue or burn off any surviving plants. This may require the use of glyphosate to burn off small patches of crop and weeds where brome plants are present.

Successful rogueing is difficult due to the sprawling nature of the plant.

Long-term successful control of sterile brome will require the removal of this weed from hedgerows. This is more challenging but as long as there are seeds present, they will be dragged back into fields by combines and cultivators, keeping the problem alive.

The trick is to prevent seed return but this cannot be done during the cropping season unless there is a buffer strip left by the hedgerow to allow the topper or hedge trimmer in during the heading out stage.

You may need to throw some perennial grass seeds into these infested areas to help colonise the base of the hedgerow when the brome plants die off.

Some farmers spray the hedges with brome-active sprays during the crop season to help prevent them from earring out and setting seed.

This can be reasonably successful at preventing normal seed-set but these plants will often survive due to the lack of completion from other plants.

Where they do not die, they may set seed later in the season and this second flush of vegetation should be flailed with a hedge trimmer after harvest if new ears appear.

Active management of hedgerows is essential if you are managing the field to eliminate brome, an outcome which is feasible.